Ian Roberts, Prof. of Linguistics, Univ. of Cambridge. The Wonders of Language (2017 1 ed).
p. 27 Middle
Be sure to get your terminology right: phonetics is the study of the speech sounds themselves; phonology is the study of how languages organise speech sounds into structured systems. To the extent that everyone has the same organs of speech (and the same perceptual apparatus, something we did not really go into in the previous chapter), phonetics is the same everywhere. But different languages may well deploy the same sounds differently, so English phonology and French phonology may be, and in fact to a fair extent are, somewhat different.
p. 82 Top
As we’ve seen, phonetics is about the sounds of language, phonology is about how those sounds pattern in linguistic systems [...]
I know that phonetic transcriptions often contain redundant information. E.g., for English, /r/ is a semivowel ([ɹ]), which we don't need to distinguish from a trilled /r/ ([r], which doesn't exist in English).
But I still don't understand why this redundancy warrants phonemic transcription? Isn't it easier to stick to phonetic transcription and stomach the redundancy? I don't know why, but I prefer phonetic transcriptions, as they spare me the extra step of contemplating which phone fits a phoneme in a particular language.